Mining method



Feb. 22, 1966 B. P. EDMoNDs ETAL 3,236,564

MINING METHOD 2 Sheets-Sheet 2 Filed July 16. 1964 FIO. 3

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D E B ROCK United States Patent O 3,236,564 MINING METHOD Byron P. Edmonds and James B. Dahms, Regina, Saskatchewan, Canada, and Edward Phelps Helvenston, Corpus Christi, Tex., assiguors to Pittsburgh Plate Glass Company, Pittsburgh, Pa., a corporation of Pennsylvania Filed July 16, 1964, Ser. No. 383,124 5 Claims. (Cl. 299-4) This application is a continuation-impart of copending application Serial No. 179,420, tiled March 16, 1962, now abandoned, which was a continuation-impart of U.S. Patent No. 3,096,969, led May 26, 1961.

This invention relates to a novel method of solution mining. It more particularly relates to controlling the size and shape of a solution mining cavity.

In solution mining a soluble, techniques are employed whereby a cased borehole is sunk down through the earth to communicate with a soluble deposit. The deposit typically comprises product minerals, i.e., minerals which it is desired to recover. Solvent of the mineable rnaterial is passed through the borehole into the deposit to extract the material contained therein. The resulting solution is withdrawn to the surface of the earth thereby establishing a cavity in the deposit. These techniques have been employed, for example, in solution mining sodium chloride, potassium chloride, sylvanite, trona, borax, and similar soluble salts.

The geometry of a cavity often significantly affects the solution mining operation. Often it is preferable that the ratio of surface area to volume of a solution mining cavity be within prescribed limits. A properly shaped cavity provides this prescribed ratio. lt is frequently desirable for a cavity to develop in the horizontal rather than in the vertical direction. In this fashion, the cavity remains in a desired strata of minerals. Moreover, there is less tendency for solvent to short circuit in a relatively shallow cavity than in a deep cavity. By short circuit is meant the tendency of solvent entering the cavity to pass to the withdrawal hole without appreciable contact with the surface of the cavity. There is often a natural tendency for the .soluble salts at the oor or the roof of the cavity to be extracted at a more rapid rate than is desired. Thus, the cavity tends to grow undesirably in the vertical direction. This invention provides a method of protecting (insulating) the floor and/ or the roof of a solution mining cavity from extraction thereby encouraging horizontal growth of the cavity. A1- though this invention is applicable to solution mining cavities in general, it is described herein with primary reference to cavities developed in the solution mining of potassium` chloride.

Potassium chloride usually occurs in mineral deposits closely associated with sodium chloride. In many cases, potassium chloride exists in admixture or in combination with `sodium chloride in the form of potassium chloride-rich strata. Often, potassium chloride-rich strata (containing l5 to 60 percent by weight of KCl based upon the total weight of KCl and NaCl in the strata) are disposed immediately above other strata which are lean as to potassium chloride, i.e., contain less than l5 percent KCl by weight based upon the total weight of NaCl and KCl therein, or which contain no substantial amount of potassium chloride but which are preponderantly sodium chloride. These mineral deposits usually contain other materials, generally clays and salts, such as calcium sulfate, magnesium sulfate and the like in small quantities, typically, 2 to 1S percent.

Subterranean deposits of potassium chloride and so diurn chloride of the above type frequently are very deep.

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For example, Canadian deposits of this character are often found 3,000 feet or more below the surface of the ground.

Recovery of potassium chloride by Water extracting subterranean deposits containing potassium chloride has not achieved commercial proportions. Serious diiculty is encountered in establishing a proper cavity because of the slow rate of extraction of KCl from the deposit, and also crystals of both potassium chloride and sodium chloride tend to form in the cavity or the cased bored hole through which the liquor is circulated, thus rendering circulation of solvent (including water and aqueous solutions capable of dissolving the mineable mineral) and efliuent in and out of the boreholes and cavity difficult.

This invention provides a method for developing a cavity with suicient surface area for the extraction of potassium chloride from a potassium chloride-rich strata where the potassium chloride-rich strata is disposed immediately above a sodium chloride strata lean in potassium chloride. According to this invention, inert liquid fluids immiscible with water are fed to a cavity being developed in the sodium chloride-rich strata. The iluids may be either more or less dense than the feed water at the temperature of operation. Fluids more dense than water form a layer at the floor of the cavity. Those less dense than water form a layer at the roof thereof. These layers insulate the floor and/ or the roof of the cavity from extraction thereby causing the cavity to expand laterally. After the cavity in the sodium chloride-rich deposit has been increased to a substantial size, the introduction of fluid to the roof thereof is partially or totally discontinued. As the cavity expands further in the lateral direction, the protective layer at the roof of the cavity be comes quite thin, thereby allowing extraction of potassium chloride from the potassium chloride-rich strata along the roof of the cavity. Thus, extraction of the potassium chloriderich strata takes place along a large dissolving surface.

In the practice of the process herein contemplated, a borehole is drilled through a potassium chloride-rich stratum or deposit and downwardly into a zone in which the potassium chloride concentration is low, i.e., below l5 percent based upon the weight of KCl and NaCl, or is substantially non-existent and where the sodium chloride is comparatively high. At this point, water or an aqueous solution of sodium chloride which is unsaturated with sodium chloride is caused to ow down the hole either through a pipe disposed in the hole or through the concentric area within the cased hole but outside the pipe. Sodium chloride is thus extracted from the potassium chloride-lean, sodium chloride-rich strata upon contact with the solution entering the strata and a cavity is established in the conventional manner.

In order to cause the cavity to expand laterally and achieve the desired size, a water-immiscible inert liquid fiuid, which has a density lower than that of water at the temperature of the operation, such as mineral oil, crude or refined petroleum oil, or like liquid hydrocar bon, is fed into the cavity in order to establish a thin layer at the roof thereof. This causes the cavity to expand laterally with a substantially iiat roof as the water is fed into the hole and the sodium chloride is dissolved essentially at the sides but not at the top of the cavity. Such liquids are found exceedingly desirable for this purpose for they give very good control over lateral expan sion of the cavity whereas gaseous iluids, for example, air or nitrogen, tend to form a dome--shaped cavity which is significantly less desirable for good expansion along the strata, i.e., does not allow for the development of a cavity of the large size desirable for the commercial extraction of KCl.

After the cavity in the sodium chloride-rich, potassium chloride-lean deposit has been increased to a substantial size, for example, when the cavity has a minimum diameter of about 15 feet, preferably at least 25 feet (as computed by measurement of the weight of volume of sodium chloride or like salt which has been removed from the deposit and/or utilizing established measuring techniques for determining the depth and width of subterranean cavities), the level of the roof is gradually allowed to rise. This is accomplished by partially, periodically or totally discontinuing the introduction of oil or like Huid into the borehole with the incoming Water. As a consequence, further extraction continues by virtue of the water flow into the hole and the withdrawal of aqueous solution therefrom until the protective layer of organic fluid becomes so thin that it ceases to prevent extraction of salt from the roof of the cavity. Gradually, then, the roof of the cavity is allowed to rise until it reaches the potassium chloride-rich strata located above the strata being mined. After the cavity has penetrated the potassium chloride-rich strata to a convenient depth, for example, several inches or more, the oil or other like fluid, which has a density less than that of water, is fed into the cavity along with water and the extraction of Ithe potassium chloride-rich strata is then commenced. This can be detected by an increase in the concentration of potassium chloride in the ellluent from the hole.

The effluent thus obtained then rises in potassium chloride content and an operating equilibrium is reached. An effluent containing about 4 to 60 pounds of potassium chloride and l5 to 36 pounds of sodium chloride per 100 pounds of water is obtained. The exact composition of the eiiluent will vary considerably, depending upon the temperature, composition and size of the strata, as well as the flow rate of extractant. Extraction of potassium chloride from the deposit can be continued by feeding water down the hole either as such or as a solution which is unsaturated both as to potassium chloride and sodium chloride. The average level of the roof of the cavity normally is above the level of water introduction in the cavity.

The level of the cavity roof is gradually raised at a controlled rate through the KCl-rich deposit by control of the level of water introduction into the cavity and the amount of mineral oil or like hydrocarbon liquid fed therein. This rate should not be in excess of about l foot of increase in level of roof per 100 to 5,000 tons of KCl withdrawn from the cavity, but should be rapid enough to ensure production of suicient strength KCl solutions.

By following the above process, the difficulties previously encountered in potassium chloride extraction are substantially eliminated. Any impurities which may be insoluble in the cavity solution can settle to the bottom of the cavity without disturbing or hindering contact of the incoming solvent with the KCl deposit and removal of cavity solution from the cavity. Moreover, the cavity is large enough to accommodate a body of solution so large that any cooling of the solution which tends to take place as a consequence of the extraction of potassium chloride is minimized by the already large body of solution in the cavity. Thus, localized cooling and consequent crystallization and plugging of equipment is minimized. Furthermore, the face of the KCl-rich cavity undergoing extraction is then large enough so that recovery of potassium chloride by the extraction process can be effected at a practical rate.

It will be understood that the rate of solution of potassium chloride is comparatively slow. For example, it is frequently found that this rate is in the range of 0.2 to 0.8 pound per hour per square foot extracting surface at a practical operating temperature, for example, 45 C. This means that extraction of enough potassium chloride to develop a cavity where practical recovery of potassium chloride is attainable is a problem. On the other hand, by establishing a large cavity in the sodium chloride-rich strata, a large surface can be established that,`

despite the slow rate of potassium chloride extraction, an effluent having a practical concentration of potassium chloride can be recovered.

This invention is diagrammatically illustrated in the accompanying drawings. FIGURE 1 shows a typical cavity, cased borehole and pipe arrangement. FIGURES 2 and 3 show typical cavities in communication with two boreholes. As shown in FIGURE l, a borehole suitably fitted with a casing 1 is drilled through the bed rock into a subterranean deposit, through a potassium chloride-rich layer and into a potassium chloride-lean, sodium chloriderich layer. A potassium chloride-rich layer may have the following approximate composition:

Percent by weight KCl 16 to 60. Water insoluble clay About l to 5. Calcium sulfate 1 to 5. Calcium land magnesium water soluble salts About 2. NaCl Remainder.

A potassium chloride-lean or sodium chloride-rich deposit may have the following typical composition:

Percent by weight There is then disposed a pipe 2 concentrially within the casing 1 of the hole. Water is then caused to ow down the hole in order to extract sodium chloride from the deposit. In the embodiment, as illustrated, water is flowed downwardly in the space between the pipe and the casing and substantially saturated sodium chloride is withdrawn from the lower part of the cavity as it is formed through pipe 2. An immiscible fluid which has a density lower than that of water and which is insoluble in or immiscible with water (preferably hydrocarbon oil) is fed in small amounts (usually in amounts up toabout 10 pounds of such agent per cubic foot of salts withdrawn) into the hole along with the water. As a consequence, this uid forms a protective layer 6 at the upper portion of the cavity 8 which is produced.

The amount of such fluid which is introduced should be enough to establish a layer of 1/2 to 8 inches in thickness at the top `of the cavity in order properly to protect the roof thereof. This amount can be computed roughly by estimating the approximate volume of the cavity from the number of tons of sodium chloride which is extracted from the deposit. Usually, about 0.1 to 2 pounds of hy-s drocarbon oil is fed per cubic foot of salts withdrawn.

In general, it is not necessary to drill into the sodium chloride-rich layer to any great depth. Usually extraction of NaCl from the NaCl-rich deposit is conducted at a level 1 to 15 feet below the level 0f the potassium chloride-rich deposit which it is ultimately desired to extract.

As a consequence of the operation, water is caused to flow rapidly into the hole and a solution of sodium chloride withdrawn therefrom, and the cavity enlarges lateral- 13 to one of substantial size, for example, preferably 25 feet or more in diameter.

After the cavity has been so enlarged, the level is raised, :as described above, by decreasing inow Iof oil to the cavity and continuing introduction of water and withdrawal of eluent. The level of water introduction usually is raised at the same time so that water owsy outwardly from the borehole casing approximately at the upper portion of the pool of cavity solution. This may be done by cutting holes in the borehole casing or by raising the casing. Finally, the level of the roof of the cavity is allowed to rise to the point where extraction of potassium chloride from the KCl-rich strata is allowed to commence. Thereafter, the KCl extraction is conducted while controlling the level of the cavity roof so that it rises at a very gradual rate through the KCl-rich deposit as described above or step-wise by successively cutting the casing or by raising it.

According to a preferred embodiment of the invention, it is desired to effect extraction of potassium chloride through a plurality of cased holes. Thus, it is more desirable to conduct the extraction by feeding water or a partially unsaturated aqueous solution of NaCl and/or KCl down one or more holes to the cavity and withdrawing the resulting KCl-sodium chloride solution from one or more other holes communicating with said cavity. This is accomplished as diagrammatically illustrated in FIG- URE 2. As shown therein, two yholes 11 and 21 are drilled and 4developed substantially, as has been described above, by establishing cavities in the sodium chloride-rich deposit. Extraction of the sodium chloride solution from the sodium chloride-rich deposit is continued from one or from both of the holes until the cavity 8 has been caused to expand laterally to the point where it is in communication with both holes. The level of the cavity is then allowed to rise through the potassium chloride-lead, sodium chloride-rich deposit until it contacts the potassium chloride-rich deposit. There is then established the cavity 8 as diagrammatically shown in FIGURE 2. This cavity has a thin layer of the inert, immiscible iluid 6 comparable in character to the layer discussed in connection with FIGURE 1.

When the holes are in communication and have been raised to a point where the roof of the cavity is above the bottom of the KCl-rich deposit, water is fed down hole 11 and the potassium chloride-sodium chloride solution is withdrawn from hole 21. The KCl-NaCl solution is withdrawn from hole 21 usually at a level below that at which water is intro-duced through hole 11 often at or near the bottom of the cavity but generally above the level where crystals or insoluble impurities have accumulated to an appreciable degree. Alternatively, .a solution of sodium chloride and potassium chloride, which is unsaturated as to both sodium chloride and potassium chloride, may be fed d-own the hole. In any event, whether water or a sodium chloride-potassium chloride solution is fed down hole 20, care generally is taken to extract both sodium chloride and potassium chloride substantially in the proportions that they exist in the deposit. This can be eiected conveniently simply by balancing the sodium chloride coutent of the solution going into hole 11.

As has been previously mentioned, potassium chloride absorbs heat when it is dissolved in water. To compensate for this, it is desirable that the temperature of the water or sodium chloride-potassium chloride solution fed down hole 11 be at least l0 to 40 F. higher than the temperature of the potassium chloride solution in the cavity. By this means, heated solution is supplied to the pool of solution in the cavity 8 and thus undue cooling of this solution is prevented.

Also, the layer 6 of inert immiscible iluid is established and maintained by feeding up to about 10 pounds of mineral oil or like liquid hydrocarbon per cubic foot of salt removed. Such normally liquid hydrocarbons are most desirable for establishing protective layer 6 for a number of reasons. In the irst place, these liquids more effectively resist extraction of the roof and, therefore, prevent an excessive rate of roof extraction.

The above process can be continued until the level of 'the cavity has been raised to the top of the potassium chloride seam and until the cavity has been expanded as far as practicable. Frequently, `it is possible to continue the extension of the cavity laterally until the amount of salt extracted therefrom indicates extraction of a cavity having a radius of 200 to 500 or more feet measured from the cased hole. Of course, it is very dificult to measure exactly a cavity of this character. However, it is possible `to estimate the size of the cavity in terms of the amount of sodium chloride and potassium chloride removed therefrom and the known composition and density of the deposit.

For most purposes, it is desirable to extract both sodium chloride and potassium chloride from a deposit substantially in the proportions in which these two materials exist in the deposit. This is desirable in order to avoid the possibility that the rate of aqueous dissolution of the deposit decreases to an impractical degree. However, after .the cavity has become very large, for example, after the size of the cavity of extreme width or lateral extension has reached 10,000 cubic feet, as computed from the volume of sodium chloride and potassium chloride :and like salts removed from the deposit, it is then possible in many cases to reduce the amount of sodium chloride being extracted. -By this time, the cavity is large enough so that extraction of potassium chloride from the sodium ichloride deposit can be effected at a practical rate by feeding aqueous solution which is saturated as to NaCl but unsaturated as to KCl into the cavity. Often, even in such instances, 5 to 50 percent of the sodium chloride existing in the deposit can be extracted from the deposit with the potassium chloride, leaving .the remaining 50 to percent of the sodium chloride in the deposit. This, of course, can be accomplished by feeidng to the cavity a solvent .solution which contains sodium chloride but is unsaturated as to NaCl yand KCl and in which the KCl to NaCl ratio is lower than that of the deposit.

It will be understood that the process herein contemplated is subject to numerous variations. For example, the cavity used to commence dissolution of the potassium chloride may be vformed by means other than extraction.

hus, a cavity may be excavated or formed by fracturing in the lower sodium chloride-rich layer or stratum or in the lower por-tion of the potassium chloride-rich stratum and then the roof of the cavity gradually raised through the potassium chloride-rich stratum by extraction .as described above.

In addition, lateral expansion may be further enhanced by the addition of a liquid heavier than water, i.e., one that has a greater density than water, to the cavity in combination with the incoming water. This embodiment is illustrated by FIGURE 3. Such liquids include halogenated hydrocarbons, such as perchloroethylene or tricholoroethylene or a rubber latex emulsion. The latter emulsion can be broken by the presence of the extracted salts to cause the formation of a layer of coagulent on the iloor of the cavity. The heavy liquid so added serves to protect the floor 4of the cavity from extraction if such is found desirable. In those cases, a l/2 to 2-inch layer 16 of the liquid or rubber crumb (coagulent) on the floor of the cavity is suicient. FIGURE 3 of the drawings .shows this heavy non-solvent layer 1-6.

Although the present invention has been described with reference lto specitic details of certain embodiments thereof, it is not intended that such details sho-uld be regarded as limitations upon the scope of the invention except insofar as included in the accompanying claims.

We claim:

1. In the method of recovering a product mineral from a subterranean deposit thereof by establishing a cavity in contact therewith and extracting the mineral therefrom with a solvent, the improvement which comprises introducing with the solvent a nonsolvent liquid more dense than the solvent at the temperature of operation and immiscible therewith to form a protective layer on the hoor of the cavity and passing said solvent through said cavity to enlarge said cavity in a lateral direction above said iloor covered by said non-solvent liquid.

2. In the method of recovering a product mineral from a subterranean deposit thereof by establishing a cavity in contact therewith and extracting the mineral therefrom with a solvent, the improvement which comprises introducing with the solvent suicient nonsolvent liquid more dense than the solvent at the temperature of operation and immiscible therewith to form la protective layer 1/2 to 2 inches thick on lthe floor of :the cavity and passing said solvent through said cavity to enlarge said cavity in a lateral direction above said floor covered by said non-solvent liquid.

3. A method of recovering a water-soluble mineral from a subterranean deposit thereof which comprises establishing la cavity in contact with said deposit by simultaneously passing water, a water immiscible nonsolvent liquid having a lower density than water and a water immiscible nonsolvent liquid having a higher density than Water to said cavity until protective layers are formed in the uppermost portion of said cavity by said liquid of a density lower than water and the lowermost portion of the cavity by said liquid of a density greater than water, each of said water immiscible nonsolvent liquids being nonsolvents ofthe salts present in said cavity, feeding Water and said nonsolvent liquid with a density lower than water into said cavity thereby dissolving said watersoluble mineral to form an aqueous solution thereof withdrawing aqueous' solution and gradually raising the level of said roof as extraction proceeds by periodically reducing theamountvof nonsolvent liquid with density lower than water passed to said cavity.

4. A method of recovering potassium chloride from a deposit having a potassium chloride-rich stratum and containing sodium chloride disposed above a sodium chloride deposit in which the potassium chloride content is relatively lower than the potassium chloride-rich deposit which comprises establishing `a cavity at least 25 feet in diameter adjacent the lower part of said potassium chloride-rich stratum wit'h an upper face in contact with said stratum -by simultaneously passing water, a water immiscible nonsolvent liquid having a llower density than waterl and a water immiscible nonsolvent liquid having a higher density than water to said cavity until protective layers are formed in the uppermost portion of said cavity by said liquid of a density lower than water and in the lowermost portion of the cavity by said liquid of a density greater than water, each of said water immiscible nonsolvent liquids being nonsolvents of the salt-s in said cavity, feeding water and said nonsolvent liquid with a density lower than water into said cavity thereby dissolving potassium chloride to form an aqueous solution thereof withdrawing said aqueous solution and gradually raising the level of said roof as the extraction proceeds by periodically reducing the amount of nonsolvent liquid with density lower than water passed to said cavity.

5. The method of recovering potassium chloride from a deposit having a potassium chloride-rich stratum containing sodium chloride disposed above a sodium chloride deposit in which the potassium chloride content is relatively lower than in potassiumA chloride-rich vstratum which comprises establishing a cavity in the sodium chloride deposit at a depth from 1 to 15 feet below the potassium chloride-rich deposit, introducing water and a water immiscible, nonsolvent liquid less dense than water at the temperature of operation to said cavity whereby to establish a thin protective layer at the top of the cavity, introducing sufficient water immisciblev nonsolvent liquid more dense than water at the temperature of operation to form a protective layer 1/2 to2 inches thick on the oor of the cavity, each of said water immiscible nonsolvent liquids being nonsolvents of the salts in said cavity, continuing to feed water and amounts of said water immiscible nonsolvent liquids to the cavity suicient to maintain said protective layers while continuing to extract the sodium chloride deposit thus causing the cavity to expand laterally with a substantially flat roof until the cavity is at least 25 feet in diameter, reducing the amount of water immiscible nonsolvent liquid with density less than water introduced to the cavity thereby extracting along the roof of the cavity so that the roof of the cavity is caused to rise into the potassium chloriderich stratum and thereafter extracting potassium chloride from the potassium chloride-rich stratum.

References Cited by the Examiner UNITED STATES PATENTS 2,009,535 7/1935 Trump 299-5 2,618,475 11/1952 Butler 299-5 2,787,455 4/ 1957 Knappen 299-5 2,822,158 2/1958 Brinton 299-5 X 3,096,969 7/1963 Edmund et zal, 299-5 X CHARLES E. OCONNELL, Primary Examiner. BENJAMIN HERSH, Exammer. 

1. IN THE METHOD OF RECOVERING A PRODUCT MINERAL FROM A SUBTERRANEAN DEPOSIT THEREOF BY ESTABLISHING A CAVITY IN CONTACT THEREWIITH AND EXTRACTING THE MINERAL THEREFROM WITH A SOLVENT, THE IMPROVEMENT WHICH COMPRISES INTRODUCING WITH THE SOLVENT A NONSOLVENT LIQUID MORE DENSE THAN THE SOLVENT AT THE TEMPERATURE OF OPERATION AND IMMISCIBLE THEREWITH TO FORM A PROTECTIVE LAYER ON THE FLOOR OF THE CAVITY AND PASSING SAID SOLVENT THROUGH SAID CAVITY TO ENLARGE SAID CAVITY IN A LATERAL DIRECTION ABOVE SAID FLOOR COVERED BY SAID NON-SOLVENT LIQUID. 